1. Field of the Invention
The present invention relates to a pixel interpolation technique adopted in an image processing circuit provided for a digital camera or the like.
2. Description of the Background Art
An imaging device such as CCD or CMOS used in a digital camera or the like photoelectrically transfers light received through a color filter to output a pixel signal. Such color filter includes an RGB color filter, a YMCK color filter and the like. A single-chip imaging device outputs a single-color pixel signal per pixel. For instance, when employing the RGB color filter, a pixel signal having one of R (red), G (green) and B (blue) color components is output per pixel.
Therefore, a pixel signal output from a single-chip color imaging device needs to be interpolated with pixel signals of other color components. Various algorithms are used in such interpolation. An exemplary method is to calculate a correlation in the horizontal direction and a correlation in the vertical direction to execute pixel interpolation using pixels present in either direction having a higher correlation. An alternative method is to execute pixel interpolation after assigning weights according to the distance between a target pixel and its neighboring pixels.
Japanese Patent Application Laid-Open No. 2001-292454 describes judging whether a pixel is in a gray edge or a color edge and determining a weighting coefficient by pixel interpolation based on the result of judgment to reduce color aliasing. Japanese Patent Application Laid-Open No. 2002-300590 describes generating color-difference signals by two methods after execution of pixel interpolation, and selecting either of the color-difference signals based on a chroma value to reduce color aliasing resulting from the pixel interpolation.
However, a conventional interpolation method causes in some cases degradation in picture quality due to misinterpolation in a gray image containing high-frequency components such as thin lines in the horizontal or vertical direction. Particularly, an image positioned on the boundary between a gray image and a color image is degraded in picture quality.